Mental-test scores and measures of school achievement often show a downward drift during childhood for low-SES children. Many reasons have been advanced to explain the decline, but the common refrain seems to be that detrimental qualities in the home environment often fail to promote mental development. There are, however, individual differences in terms of which child is more susceptible to these detrimental effects, or by contrast which one is better buffered and able to make substantial progress in spite of an impoverished environment. The role of intrinsic characteristics as modifiers of environmental impact would seem to be potentially important, although relatively little attention has been devoted to this topic. The current research project is recapturing a sample of low-SES twins who were originally recruited for a longitudinal study of mental development but became inactive at 9 years. Most of the twins had completed a battery of mental tests from birth to 9 years, were reactivated for their 12th birthday during 1984-1987, and were assessed for intelligence, school achievement, and temperament at 12 years. A home visit had also been carried out for the families, and parents provided self-assessments of their own personality and temperament which afforded ratings of home and parents as effective agents for fostering mental development. The proposed continuation of the research project will enlarge the sample to include about 100 twin pairs for the criterion measures at 12 years, and about 100 twin pairs of 15-year-olds as a comparison group. All data will be subjected to multivariate analyses (including path analysis) to identify the links between early mental development home-and-parental variables, and the criterion measures at 12 years. The project capitalizes upon a pre-existing data source based on twins to help clarify whether perinatal risk and genetic factors, as well as child-individual differences variables, play a role in producing the range of individual differences in school achievement.